Generally, in a case where a variety of operations such as spray coating, paint applying operation, arc welding or the like are performed using an industrial robot, such a significant requirement that an arm of the industrial robot holding a tool such as an injection nozzle or the like should approach closer to an object (for instance, a vehicle) without an occurrence of collision with the object is imposed on the arm of the industrial robot. In some case, an object is permitted to move relative to the tool carried by the arm of the industrial robot during operation. In this case, the robot arm should have a possibility that it assumes a positional attitude which enables the object to move freely. To assure properties which satisfactorily meet the aforementioned requirement, an industrial robot is required to have a degree of freedom more than that required for positional displacement (translational movement) of a tool and rotation (adjustment of angle) of the same, that is, mobility. When it is assumed that a position and angle should be completely specified for a tool, there is a need of giving six degrees of freedom to the tool. By doing so, it follows that the industrial robot should have at least seven independent movements in view of the aforementioned requirement. A fact that the industrial robot has an extra degree of freedom means that it can be controlled so as to enable the tool to be held at a required relative position as well as a required relative angle with respect to an object, for instance, during rotation of the arm for enabling the latter to be kept away from some obstruction.
In addition, reachability of a certain industrial robot (properties relative to a range of operation) is dependent on an ability of movement of the industrial robot. In other words, individual components constituting the industrial robot are subjected to a geometrical restriction in the form of crosswise connection between movements of individual arm portions and mechanical final position.
Translational movement of a tool is performed mainly by means of the robot arm, while angular adjustment (rotation) of the tool is achieved by actuating a hand link on the robot. Usually, an industrial robot has three degrees of mobility and a hand link has two or three degrees of mobility. It is desirable that the hand link performs a possibly small amount of translational movement. On the other hand, it is preferable that the arm does not perform an excessive amount of rotation during a small amount of translational movement of the hand link.
Most industrial robots have five or six degrees of mechanically independent movements. For the purpose of realizing required reachability (properties relative to a range of operation), a robot is often placed on a moving track and this leads to a result that the robot can have mobility increased by one degree or in some case, by two degrees. Another measure for solving a problem concerning required reachability (properties relative to a range of operation) is to displace an object relative to a robot using, for instance, a rotational table and/or raising and lowering the table.
FIG. 1 is a perspective view illustrating a conventional industrial robot having eight degrees of mobility.
With respect to the conventional industrial robot, reachability (properties of a range of operation) can be increased by both a moving track and a rotational table.
A thing in common with measures for realizing the aforementioned reachability (properties relative to a range of operation) required for an industrial robot is that all movements should be simultaneously controlled while maintaining independence of mechanical operations irrespective of those inherent to the robot or those given from the outside.
Indeed, it is desirable that an industrial robot has wide versatility and it is preferable that the number of extra accessories or components is reduced to as few as possible. However, the aforementioned requirement for realizing reachability (properties relative to a range of operation) required for an industrial robot has been heretofore met by producing a robot which has the number of movements more than that the number required for duties to be executed. Indeed, an evaluation to be made with respect to the so obtained reachability (properties relative to a range of operation) depends on where and how a movement considered as a target has been achieved.
Overwhelmingly many kinds of robot arms which have been heretofore produced are constructed in a so-called shaft-shaped structure (or shoulder-shaped structure). With respect to robot arms as constructed in this way, a plurality of serially arranged rigid arm segments are supported on a single base on which the arms can be rotated as required. In a case where arms having the aforementioned shaft-shaped structure substantially similar to arms usable for excavating bucket, crane or the like are employed, among a plurality of joint axes along which the respective arms are operatively connected to one after another, at least two axes normally extend in parallel with one another. With such construction, reduction of an increase in certain partial reachability is intended for the purpose of increasing reachability as a whole. In a case where the shaft-shaped structure is employed, an ability of movement is often restricted in order to assure that at least two arm portions operate in an identical plane. This leads to a result that a robot having the aforementioned shaft-shaped structure to effect an increase in reachability is often permitted to have an unnecessarily number of extra movements at a location where a conventional shaft type structure is supplemented with a number more than a mathematically required number of freedom.
Apart from the foregoing shaft-shaped structure, another type of shaft-shaped structure (shoulder-shaped structure) which is employed for a certain kind of work to bring good results with respect to an increase in reachability is such that it includes a series of arms adapted to be successively bent. Namely, the arms do not have any discontinuous joint point. Such a robot arm as mentioned above is constructed by a plurality of elements adapted to directly or indirectly come in rolling contact with one another and these elements are connected to one after another using an actuating element by way of which power is transmitted, for example, a thread. Swedish Patent Application NO. 8304726-6 discloses an embodiment for such a robot arm having seven degrees of mobility or movement. However, it has been found that a principle of operation of an industrial robot explained in the prior invention for which a patent application was filed is restrictively utilized only to an operation with a low accuracy required therefore, such as spray coating or the like, due to weak points involved in the principle and frictional hysteresis.
In addition, it is desirable that an industrial robot is electrically driven using transmission elements having a possibly small number of serial connections and converting elements.
Additionally, there is a requirement that robot arms and a hand link should be hollow to form a passage in which a conduit and a cable are provided at a position located close to a symmetrical axis of the robot. This passage should have a large radius of curvature when the arms and the hand link are bent to a maximum limit. Further, they should be so constructed that conduit and cable are handled carefully.
An ability of orientation of the hand link has been improved in accordance with the same method as that for increasing reachability (properties relative to a range of operation) with respect to an industrial robot. Particularly, the shaft-shaped structure is employed at a location where rotational axes of the hand link intersect each other and thereby an ability of orientation can be obtained by forming a rotational surface which is inclined with respect to the symmetrical axes of the hand link. For the purpose of increasing an ability of orientation of the hand link to accomplish an identical object, in other words, for the purpose of rotating a tool with respect to an object, intersections on the axes should be located possibly close to each other. By employing this method, it is possible to form a rotational point with respect to the tool by a small amount of translational movement or without any translational movement. The Swedish Patent Application No. 8304726-6 discloses a hand link having a high intensity of rotational ability and a small amount of translational movement with respect to a rotational point on the tool.
The present invention has been made with the foregoing background in mind and its object resides in providing a robot arm usable for an industrial robot which assures an extremely high ability of movement, excellent reachability and high accuracy with the aid of direct electrical driving and a central passage through which conduit and cable extend.